This invention relates to a method of making charge coupled devices (hereinafter CCD's) of the type where excess charge is removed from the potential wells in a channel without spilling over to an adjacent well in that channel and, more particularly, to a method of making such devices with buried channels.
CCD's are electronic devices that store and transfer charge along a channel. They generally include a semiconductor substrate over which is formed an insulating layer as well as parallel electrodes that extend over the layer in a direction generally transverse to the channels. Between the channels, there are provided channel stop diffusions that form potential barriers to confine the charge in its channel by preventing it from spilling over to the adjacent channel. The electrodes are used to control potential wells in the substrate adjacent each electrode. By sequentially applying clocking signals, charge can accumulate in these wells and can be transferred from one well to the next along an individual channel.
One of the problems associated with CCD's is charge overload. For example, if the CCD is used in an image sensing array to store and transfer radiant energy and if an intense radiant energy source is imaged onto a particular well, the amount of charge generated at that location may be in excess of that which can be stored. This excess charge is prevented from leaving the channel because the surface potential created by the channel stop is lower than the surface potential between adjacent wells along a channel. Thus, the excess charge spills out of its potential well and into one or more adjacent wells in the same channel. This phenomena results in the spreading of any intense image read out of the array and is known as "blooming".
A solution to the charge overload or "blooming" problem is disclosed in U.S. Pat. No. 3,863,065 issued Jan. 28, 1975 to W. F. Kosonocky et al. This solution includes a drain in the form of diffusions in the center of each channel stop and a more lightly doped channel stop such that its potential barrier is less than that between wells along the channel. Excess charge spills over the lower potential barrier formed by the more lightly doped channel stops into the drain. The drain is connected to a voltage source and, thus, the excess charge is carried away.
When making buried channel CCD's in accordance with the Kosonocky et al. patent, a semiconductor wafer is first masked and then a first diffusion step is performed to form the drains. Thereafter, the masking material is stripped from the wafer and a second masking and diffusion step is performed to form the channel stops. At this point, the second mask is stripped from the wafer and a third masking and diffusion step is performed to form the channel regions. The numerous masking and diffusion steps add significantly to the cost of manufacture. Moreover, mask alignment problems between the drains and the channel stops are severe and even the slightest misalignment results in a distortion of the image. This also results in a significant loss of yield and adds to the cost of manufacture.